Allow stubbing all the way at the root of ngHal

lets you do things like:

```javascript
ngHal.stubFollow('stories', ngHal.mock('http://meta.prx.org/model/story', {title: 'foo'}));
```
at that point, `ngHal.follow('stories')` (anywhere in the code) will point at your mock object.

This makes testing your `resolve`s much easier.